Energy storage system

ABSTRACT

A closed loop energy storage system configured with a hydrogen tank, an oxygen tank, a fuel cell stack and an electrolyzer. A heat exchanger freeze-dries the hydrogen and oxygen prior to their storage in their respective tanks. The heat exchanger also uses excess fuel cell heat to preheat streams of hydrogen and oxygen coming from the tanks. Phase separators serve both to separate water from hydrogen and oxygen, and to store the water. A thermal management system encloses all the system components except the tanks. An airfoil-shaped shell covers the system, and the larger of the two tanks extends substantially across the shell at its point of greatest camber thickness. The tanks are composed of polymer liners integral with composite shells.

This application is a continuation application of application Ser. No.12/653,921, filed Dec. 17, 2009, which is a continuation application ofapplication Ser. No. 12/589,914, filed Oct. 30, 2009, which is acontinuation application of application Ser. No. 11/981,202, filed Oct.31, 2007, now U.S. Pat. No. 7,611,789, which is a divisional ofapplication Ser. No. 10/418,737, filed Apr. 17, 2003, now U.S. Pat. No.7,531,254, which claims priority from U.S. provisional patentapplication 60/373,301, filed Apr. 17, 2002, each of which areincorporated herein by reference for all purposes.

BACKGROUND

This invention relates generally to methods and apparatus for storingenergy and, more particularly, to energy storage systems forhigh-altitude, light weight, long duration solar powered aircraft, andrelated methods.

Aircraft are used in a wide variety of applications, including travel,transportation, fire fighting, surveillance and combat. Various aircrafthave been designed to fill the wide array of functional roles defined bythese applications. Included among these aircraft are balloons,dirigibles, traditional fixed wing aircraft, flying wings andhelicopters.

One functional role that a few aircraft have been designed to fill isthat of a tightly stationed (i.e., maintained in a small, laterally andvertically limited airspace), high-altitude (e.g., stratospheric),long-duration platform. Operating from high, suborbital altitudes attemperatures well below the freezing point of water, such aircraft canoperate as communication relay stations between ground-based andspace-based stations. However, to have truly long flight durations, theaircraft must either carry enough fuel to last extreme lengths of timeor have the ability to refuel and/or recharge its energy storage system.

Given the broad range of functions that a long-duration, tightlystationed, suborbital platform has the potential to perform, it isdesirable to design such platforms to be capable of handling largerpayloads and power demands, which typically drain resources necessary tomaintain flight through a full range of typical weather conditions.

One potential source of power that can be used to recharge an aircraft'senergy storage is solar power. However, solar power is intermittent, andsolar-powered aircraft must have significant energy storage systems tostore enough energy to fly through each night during a flight.

Therefore, there exists a definite need for apparatus and relatedmethods for repeatedly storing and discharging energy. Preferably, usingsuch methods, such an apparatus should be able to operate up to veryhigh, suborbital altitudes for a long period of time. Importantly, it isdesirable for such apparatus to have significant energy storage capacitywhile weighing as little as possible. Furthermore, it is preferable forsuch an apparatus to be relatively inexpensive to build, maintain andoperate and, furthermore, be relatively pollution-free. Variousembodiments of the present invention can meet some or all of theseneeds, and provide further, related advantages.

SUMMARY OF THE INVENTION

In various embodiments, the present invention solves some or all of theneeds mentioned above, providing an energy storage system and relatedaircraft structures, as well as related methods.

A closed loop energy storage system of the invention typically includesa fuel cell stack configured to react a gaseous fuel with a gaseousoxidizer to produce a liquid product and electrical power, and anelectrolyzer stack configured to separate the liquid product into ahumid fuel including the gaseous fuel and product humidity, and a humidoxidizer including the gaseous oxidizer and product humidity. The energystorage system also typically includes a fuel tank configured to storedried, gaseous fuel and an oxidizer tank configured to store dried,gaseous oxidizer.

A preferred feature of the invention is a heat exchanger configured toremove the product humidity from the humid fuel and the humid oxidizerby freezing the product humidity, to produce dried, gaseous fuel anddried, gaseous oxidizer, wherein the heat exchanger is furtherconfigured to preheat the gaseous fuel from the fuel tank and thegaseous oxidizer from the oxidizer tank prior to their being reacted inthe fuel cell stack. Advantageously, this bidirectional feature willgenerally provide for weight reduction and system simplification. Theweight reduction allows for superior performance for a given set offlight parameters, and the system simplification contributes toreliability and cost efficiency.

A second preferred feature of the invention is that the heat exchangeris configured to use excess heat from the fuel cell stack to preheat thegaseous fuel from the fuel tank and the gaseous oxidizer from theoxidizer tank. This feature generally provides for energy-efficiency.

A third preferred feature of the invention is that the system mayfurther include phase separators that serve both to separate gaseousreactants from liquid reaction product, and to store the liquid reactionproduct for use in the electrolyzer. This feature generally provides fora structurally, functionally and thermodynamically simpler system.

A fourth preferred feature of the invention is that the system mayfurther include a thermal management system for product-containingcomponents but not for the storage tanks, along with the heatexchanger/drier. This feature generally provides for cost and weightefficiency.

A fifth preferred feature of the invention is that the system mayfurther include an airfoil-shaped outer shell containing the storagetanks, wherein the larger tank extends substantially across theairfoil-shaped outer shell profile at its point of greatest camberthickness. This feature generally provides for weight and aerodynamicefficiency.

A sixth preferred feature of the invention is that the system storagetanks may be composed of polymer liners integral with composite shells.This feature generally provides for weight efficiency.

Other features and advantages of the invention will become apparent fromthe following detailed description of the preferred embodiments, takenwith the accompanying drawings, which illustrate, by way of example, theprinciples of the invention. The detailed description of particularpreferred embodiments, as set out below to enable one to build and usean embodiment of the invention, are not intended to limit the enumeratedclaims, but rather, they are intended to serve as particular examples ofthe claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a solar aircraft power system.

FIG. 2A is a right side, elevational cross-section view of an energystorage system pod embodying the present invention.

FIG. 2B is a left side, elevational cross-section view of the energystorage system pod depicted in FIG. 2A.

FIG. 2C is a top plan, cross-section view of the energy storage systempod depicted in FIG. 2A.

FIG. 2D is a bottom plan, cross-section view of the energy storagesystem pod depicted in FIG. 2A.

FIG. 2E is a front elevational, cross-section view of the energy storagesystem pod depicted in FIG. 2A.

FIG. 2F is a rear elevational, cross-section view of the energy storagesystem pod depicted in FIG. 2A.

FIG. 2G is a top plan airfoil configuration drawing of the energystorage system pod depicted in FIG. 2A, showing the camber at variouschord locations.

FIG. 3A is a left, rear perspective view of internal portions of theenergy storage system pod depicted in FIG. 2A.

FIG. 3B is a right, front perspective view of the internal portionsdepicted in FIG. 3A.

FIG. 4A is a schematic diagram of the energy storage system pod depictedin FIG. 2A.

FIG. 4B is a detailed schematic diagram of a first portion of the energystorage system pod depicted in FIG. 2A.

FIG. 4C is a detailed schematic diagram of a second portion of theenergy storage system pod depicted in FIG. 2A.

FIG. 4D is a detailed schematic diagram of a third portion of the energystorage system pod depicted in FIG. 2A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an energy storage system. Features andadvantages of the invention will become apparent from the followingdetailed description of the preferred embodiments, taken in conjunctionwith the accompanying drawings, which illustrate, by way of example, theprinciples of the invention.

With reference to FIG. 1, a solar aircraft power system 101, including afirst energy storage system 103 embodying the invention, includes solarcells 105 to provide power through a power bus 107 for aircraft motors109 and other aircraft loads such as aircraft control systems 111(including the energy storage system control system) and various activepayloads 113. The solar cells also provide enough excess power to chargethe energy storage system during the day so that the energy storagesystem can provide electrical energy to operate some or all of theaircraft's electrical loads when solar power is limited or notavailable, such as at night. Preferably, the energy storage system usesan electrolyzer stack 115 and a fuel cell stack 117 to store and releasepower, respectively.

The energy storage system is electrically connected to the aircraft'selectrical power bus via DC-DC converter circuitry 119 configured toaccommodate the voltage level requirements of the electrolyzer stack andthe fuel cell stack. More particularly, a lightweight, high-efficiencybidirectional DC-DC converter is preferably used to connect the fuelcell and the electrolyzer stack to the main aircraft power bus, whichpreferably operates between 70 and 170 V DC. The DC-DC converterpreferably operates at high conversion efficiencies. Since the optimumfuel cell and electrolyzer stack voltages might differ in preferredvoltage ranges (e.g., 50-85 V for the fuel cell and 60-90 V for theelectrolyzer), a converter might be essential for connecting the stacksto the main power bus. A high-efficiency converter provides theflexibility to optimize the stack electrochemical performance andpackaging to achieve efficient, lightweight, and reliable stack designs.

It may become possible in the future to make stacks with operationalvoltages in the same range as the bus voltage. This would allow fordirect connection of the fuel cell to the bus without a DC-DC converter,resulting in a small efficiency gain. During the daytime a DC-DCconverter is still preferred. This is because during the daytime it isnecessary to provide sufficient power to the propulsion system whilekeeping the bus voltage at a value that maximizes solar array poweroutput. These two requirements need two controls to achieve. Here, thetwo controls are the motor throttle and the DC-DC voltage ratio.

Preferably an autonomous, fault-tolerant control system is used tocontrol the energy storage system pod. Dual redundant energy storagesystem control computers are preferably used in each pod to collect dataand control system components. In the event one energy storage systemcomputer fails, the second energy storage system control computerautomatically takes over control of the energy storage system. Sensorsare used to collect system health information that is used by thecontrol system to optimize performance and ensure safe operation.Predefined safety checks are automatically performed to ensure allsystem operating parameters are within limits. The system health dataare downloaded via an RF link and displayed on ground control computers.Warnings are displayed on the ground control screens when any parameterexceeds predefined limits. Automated shutdowns occur when key safetylimits are exceeded. The charging and discharging cycle is fullyautomated, with manual overrides.

Energy Storage System Pod

With reference to FIGS. 2A-2G and 3A-3B, the energy storage system ispreferably a closed loop system contained in a pod 201 that hangs downfrom the wing of the aircraft. Under the direction of its controlsystem, the energy storage system stores excess power provided byaircraft's solar array during the day and provides the power needed tooperate the solar aircraft at night. Preferably, more than one pod ismounted on an aircraft to provide additional storage capacity andredundancy, and the pods are modular. The energy storage system pods arelightweight and operate at high-efficiency, preferably enabling thesolar aircraft to fly continuously (up to six months or more) at highaltitudes, above commercial air traffic lanes and the jet stream,without landing. An example of an appropriate aircraft for using suchpods may be found in U.S. Pat. No. 5,810,284, which is incorporatedherein by reference for all purposes.

The pod defines a forward and rearward direction based on the aircraft'snormal flight orientation and the resulting relative fluid flow (i.e.,airflow past the pod). The pod 201 includes an inner frame having a mainshaft 203, and further includes an outer shell 205. The frame main shaftincludes a structural attach point 207 at an upper end of the frame mainshaft, for attaching the pod to an aircraft wing 209. The frame alsoincludes a forward wheel 211 and a rearward wheel 213 mounted on lateralframe struts 219 at a lower end of the frame, the wheels beingconfigured to bear landing loads for the aircraft.

The frame supports both the 205 shell and various components of theenergy storage system, including a fuel storage tank 215 (“the fueltank”) for accumulating and storing fuel (e.g., H₂), and an oxidizerstorage tank 217 (“the oxidizer tank”) for accumulating and storing anoxidizer (e.g., O₂). These two tanks are cylindrical tanks, and eachtank defines a tank axis along the tanks' cylindrical axis of symmetry.The tanks are preferably supported with their tank axes in a relativelyvertical orientation, as shown in FIG. 2A.

The tanks are each supported by lateral frame struts 219 that supportthe tanks at upper and lower tank ends, 221 and 223, respectively, alongthe tank axes. The fuel tank 215 is positioned forward of the frame mainshaft 203 and over the lateral frame strut 219 upon which the forwardwheel 211 is mounted. The oxidizer tank 217 is positioned rearward ofthe frame main shaft and over the lateral frame strut 219 upon which therearward wheel 213 is mounted. Optionally, the lateral frame strutssupporting the wheels can also be configured as lateral strutssupporting the lower ends of the tanks.

In this configuration, the larger of the two tanks, which is the fueltank 215 in this embodiment, is therefore mounted forward of the framemain shaft 203, which is forward of the smaller of the tanks within theshell 205. Preferably, most or all of the remaining components of theenergy storage system, including all components that at leastoccasionally contain the product of the fuel cell reaction (“the liquidproduct” or “the product”) (e.g., H₂O), are positioned proximate theframe main shaft 203, and between the forward, fuel tank and therearward, oxidizer tank 217.

These remaining components include the fuel cell stack 117 and theelectrolyzer stack 115, as well as other components that at leastoccasionally contain the product of the fuel cell reaction. Preferably,these remaining components of the energy storage system, or at least allcomponents that at least occasionally contain the product of the fuelcell reaction, are retained within a thermal enclosure. Preferably, theenergy storage system works with a thermal control system that preventswater from freezing at any point in the system except for the oxygen andhydrogen gas driers. Preferably, the fuel tank and the oxidizer tank arenot thermally enclosed.

As is clear in FIGS. 2C, 2D and 2G, the outer shell has anairfoil-shaped profile 231 in a plan (downward) view. The size and shapeof the cross-sectional airfoil-shaped profile varies downward along avertical axis of the shell, as is suggested by the shell outlinedepicted in FIGS. 2A and 2B. Therefore, the airfoil-shaped profiledefines a camber thickness that varies along a chord line 233 of theairfoil-shaped profile.

Preferably, the forward, larger fuel tank 215 extends substantially orentirely across the airfoil-shaped profile 231 at a chord location 235at or near the chord location 237 of greatest camber thickness 239. Inthis context, “substantially across” means fully across other than spacerequired for local support structure or structural requirements such asmovement clearances, and “substantially near” means at a location havinga camber thickness equivalent to that at the location of camberthickness, from a perspective of tank capacity. The rearward, smalleroxidizer tank 217 is preferably at chord location 241 having a narrowercamber thickness 243, such as a chord location having a camber thicknessinadequate for mounting the forward tank.

This configuration provides for a weight and cost efficient tank shape(e.g., cylindrical) to be used in an aerodynamic shell that is efficientfrom the standpoint of weight and drag. Furthermore, the placing of theproduct-containing devices between the two tanks provides for a moreconstant energy storage system center of gravity, which preventsinterference with aircraft flight characteristics during operation ofthe energy storage system. Additionally, with the energy storage systemcomponents, with the exception of the tanks, packaged tightly together,the thermal environment of the components can be regulated with aminimum amount of thermal insulation and its associated weight.

Energy Storage System

The design of the energy storage system utilizes and/or works withseveral interdependent subsystems to provide a lightweight,high-efficiency, reliable energy storage system that enables continuoussolar aircraft flight at high altitudes. The energy storage systempreferably has a specific energy that is greater than 400 Whr/kg. Theunique and innovative features of these subsystems contribute to theoverall operating efficiency, fault tolerance, and flexibility of theenergy storage system.

With reference to FIG. 4A-4D, in addition to the fuel cell stack 117,the electrolyzer stack 115, the fuel tank 215 and the oxidizer tank 217,the energy storage system further includes an oxidizer phase separatortank 301 (or “oxidizer phase separator”), a fuel phase separator tank303 (or “fuel phase separator”), and a preferably ethanol-based heatmanagement system 305 including a main heat exchanger 307, a ram-airheat exchanger 309, and a bidirectional drier 311 (i.e., a drierconfigured for both heating and cooling). The fuel cell stack andelectrolyzer stack are designed to operate over a wide pressure range(e.g., from 40 to 400 psig), which allows them to match the overallsystem pressure over a wide range of operating pressures.

The fuel tank 215 is configured to provide a stream of dry, gaseous fuel(e.g., H₂) to the fuel phase separator 303 via a bidirectional fuel line321 (i.e., a line where fuel can flow in both directions). The fuelphase separator is configured such that a stream of gaseous fuel can bedrawn into an anode side of the fuel cell stack 117 via a fuel inputline 323. Likewise, the oxidizer tank 217 is configured to provide astream of dry, gaseous oxidizer (e.g., O₂) to the oxidizer phaseseparator 301 via a bidirectional oxidizer line 325 (i.e., a line wherethe oxidizer can flow in both directions). The oxidizer phase separatoris configured such that a stream of gaseous oxidizer can be drawn into acathode side of the fuel cell stack 117 via an oxidizer input line 327.

As will be discussed below, the streams of dry, gaseous oxidizer and drygaseous fuel are warmed prior to reaching their respective phaseseparators. The phase separators will typically contain warm liquidproduct and unreacted humid gases, which humidify and further warm thestreams of dry gases from the storage tanks before they are drawn to thefuel cell stack 117.

The fuel cell stack 117 is configured as a proton exchange membrane fuelcell to react the stream of gaseous fuel from the fuel input line 323with the stream of gaseous oxidizer from the oxidizer input line 327 toproduce a stream of liquid product (e.g., H₂O), and electrical powerthat is delivered to the main aircraft power bus. As is describedfurther below, a fuel-cell cooling line 329 is configured to passthrough the fuel cell stack, such that liquid product can be used toremove excess heat from the fuel cell stack.

The stream of liquid product produced by the fuel cell stack 117 istypically intermixed with unreacted oxidizer. Therefore the fuel cellstack is configured such that a stream containing a mix of product andhumid oxidizer (i.e., oxidizer that passed through the fuel cell cathodewithout reaction, and that now contains evaporated liquid product) isdrawn out of the fuel cell stack via a product output line 331. Theproduct output line connects to the oxidizer phase separator, into whichthe mix stream can be pumped. The oxidizer phase separator is configuredto separate the mix stream liquid product from the mix stream humidoxidizer. The configuration of the oxidizer phase separator allows theseparated humid oxidizer to intermix with the stream of gaseous oxidizerfrom the bidirectional oxidizer line 325, to thereby recover theunreacted oxidizer for use in the fuel cell.

The oxidizer phase separator is configured with an adequate liquidproduct capacity to contain all of the liquid product generated by thefuel cell stack 117 through its intended operational limit. Preferably,the oxidizer phase separator is configured with an adequate liquidproduct capacity to contain all of the liquid product generated throughthe processing of substantially all the gaseous fuel and/or gaseousoxidizer available in the fuel and oxidizer tanks. In this context,“substantially all” refers to all the gaseous fuel and/or gaseousoxidizer other than the small quantity that would naturally remain inthe tanks and lines when the gaseous fuel and/or gaseous oxidizer supplywas essentially depleted. As a result, there is no need for the weightand expense of providing a separate storage tank for the liquid product,along with its necessary pumps, sensors, overflow valves, and the like.

The fuel cell stack 117 also emits a stream of unreacted fuel from itsanode side. The fuel cell stack is configured for the unreacted fuel tobe drawn out of the fuel cell stack via an unreacted-fuel output line333, and pumped into the fuel phase separator. The fuel phase separatoris configured such that the stream of unreacted fuel, which may be humidfuel (i.e., fuel that passed through the fuel cell anode withoutreaction, and that now contains evaporated liquid product), isintermixed with the stream of gaseous fuel from the bidirectional fuelline 321, to thereby recover the unreacted fuel for use in the fuel cellstack. If any liquid product should be emitted from the anode side ofthe fuel cell stack, the fuel phase separator is configured to separatethe liquid product from the humid fuel.

The electrolyzer stack 115 is configured as a proton exchange membraneelectrolyzer, receiving electrical power from the aircraft power bus(e.g., originating from the solar cells), and using it to separate astream of liquid product into a stream of a first mixture including theliquid product and the gaseous fuel, and a stream of a second mixtureincluding the liquid product and the gaseous oxidizer.

The oxidizer phase separator is configured to have a stream of liquidproduct drawn through a product-output line 339 and an electrolyzerinput line 341 and then pumped to the electrolyzer stack 115 duringelectrolyzer stack operation. The electrolyzer stack is configured toperform the separation on this stream of liquid product, and to pass theresulting first mixture stream out through a first mixture output line343, and the resulting second mixture stream out through a secondmixture output line 345. The first and second mixture output lines areconfigured to pass their mixture streams to the fuel phase separator andoxidizer phase separator, respectively. As noted above, each of thephase separators is configured to separate liquid product from the humidgas.

During operation of the electrolyzer stack, the fuel cell stack 117 isnot ordinarily operated, or is only operated at a very low level. Withthe exception of any oxidizer sent to the fuel cell stack, the oxidizerphase separator is configured to pass the received gaseous oxidizer backthrough the bidirectional oxidizer line 325, and on to the oxidizertank. Likewise, with the exception of any fuel sent to the fuel cellstack, the fuel phase separator is configured to pass the receivedgaseous fuel back through the bidirectional fuel line 321, and on to thefuel tank.

Over a period of operation, the fuel phase separator may containsignificant amounts of liquid fuel. The fuel phase separator isconfigured with a product-reduction line 351 for liquid product to bedrawn into the electrolyzer input line 341. Both the product-output lineand the product-reduction line have solenoid valves, 353 and 355,respectively, configured such that the electrolyzer input line streamcan be redirected to come from the fuel phase separator. The controlsystem is configured to operate these valves to that end when atriggering maximum level of liquid fuel accumulates in the fuel phaseseparator. The control system is further configured to reset thesevalves and redirect the electrolyzer input stream to be from theoxidizer phase separator when a triggering minimum level of liquid fuelis left in the fuel phase separator.

The energy management system includes the heat management system 305 toaccommodate the thermal needs of other portions of the energy managementsystem. The heat management system preferably includes two intermixedcoolant loops: a main coolant loop and a bidirectional-drier coolantloop. Preferably, the heat management system uses low freezing pointfluid coolant, such as ethanol, to exchange heat between the atmosphereand system components. Both coolant loops share the ram-air heatexchanger, which is configured to radiate excess heat into theatmosphere, and a coolant pump. Each coolant loop is configured with aseparate, controllable bypass line to bypass the ram-air heat exchanger.

The main coolant loop includes the coolant pump, the ram-air heatexchanger, a first ram-air heat exchanger bypass line 363, and the mainheat exchanger. The main heat exchanger is configured to cool liquidproduct flowing in the line leading from the oxidizer separation tank tothe electrolyzer stack and the fuel cell stack 117 (via the fuel-cellcooling line 329). The main coolant loop is thus configured to removeheat from the fuel cell stack. This heat can either be radiated via theram-air heat exchanger, or passed on to the bidirectional drier coolantloop.

The bidirectional drier coolant loop includes the coolant pump, theram-air heat exchanger, a second ram-air heat exchanger bypass line 365,and the bidirectional drier. The bidirectional drier is configured toeither heat or cool both the bidirectional oxidizer line 325 and thebidirectional fuel line 321. In a first mode of operation, thebidirectional drier coolant loop is configured to use the heat from thefuel cell stack 117 (via the main coolant loop) to warm the streams ofdry, gaseous oxidizer and dry, gaseous fuel passing through thebidirectional oxidizer line 325 and the bidirectional fuel line,respectively, on their way to the fuel cell stack.

In a second mode of operation, the bidirectional drier coolant loop isconfigured to use the ram-air heat exchanger to cool the bidirectionaloxidizer line 325 and the bidirectional fuel line 321 such that thehumidity (evaporated product) is frozen out of the streams of humidoxidizer and humid fuel, respectively. This freezing of the humidifydries the oxidizer and fuel to allow only dry, gaseous oxidizer and dry,gaseous fuel to reach the respective storage tanks. Because only dryoxidizer and fuel reach the storage tanks, little or no thermalmanagement need be provided for the storage tanks. During the first modeof operation, the frozen product in each line's passage through thebidirectional drier is thawed and returned to the respective phaseseparators.

Detailed Operation Of The Electrolyzer (Charging)

The electrolyzer stack 115 uses electrical energy and a stream of theliquid product to produce streams of the gaseous fuel and gaseousoxidizer, which are routed to and stored in the storage tanks.

In particular, the oxidizer phase separator 301 serves as a liquidproduct storage tank, and as a gas/liquid phase separator. A stream ofthe liquid product is pumped from the oxidizer phase separator throughthe product-output line 339 and the electrolyzer input line 341, andinto the oxidizer side of the electrolyzer stack 115, and is used bothas a reactant and as a cooling fluid. At the direction of the controlsystem, a variable flow, product-output line pump 401 drives andregulates the product flow through an electrolyzer input line filter 402and the electrolyzer stack. A flow gauge (FG22), temperature sensors(T25) and (T46), and pressure transducers (PT22) and (PT42) monitor theparameters of this stream for the control system.

At the direction of the control system, the main heat exchanger 307 isused to maintain the desired product temperature as it enters theelectrolyzer stack. The coolant pump 361 circulates the coolant betweenthe main heat exchanger and the ram-air heat exchanger 309 to radiateheat. A pair of main-loop proportional valves 403 and 405, together witha temperature sensor (T2), are used to control the temperature of thecoolant at the inlet of the main heat exchanger by controlling thebypass of the ram-air heat exchanger in the main cooling loop. Thiscooling loop operates at ambient pressure.

The temperature sensor (T21) is used to monitor the temperature of theoxidizer phase separator 301 and also the product input stream at aninlet to the main heat exchanger 307. A temperature sensor (T26)monitors the temperature at the exit of the main heat exchanger 307 andis used to verify the proper operation of the main cooling loop for thecontrol system. A temperature sensor (T1) and a flow gauge (FG1) areused to assess the proper operation of this cooling loop.

The first mixture output line 343 passes a stream containing a mixtureof humid fuel and product to the fuel phase separator 303, and thesecond mixture output line 345 passes a stream containing a mixture ofhumid oxidizer and product to the oxidizer phase separator 301. As notedabove, each of the phase separators is configured to separate the liquidproduct from the humid gases.

Electrolyzer oxidizer check valve 471 and electrolyzer fuel check valve473 prevent back flow of oxidizer and fuel, respectively, in the eventof a failure in the electrolyzer stack 115. The proportional,electrolyzer input valve 475 is used to regulate the flow of product tothe electrolyzer stack when it is not in use. During electrolyzeroperation, the electrolyzer input valve will be fully open, and duringfuel cell operation it may be partially open to maintain theelectrolyzer stack temperature at a desired value.

A significant amount of product is transferred to the fuel side of theelectrolyzer stack 115, and thus to the fuel phase separator 303 duringthe operation of the electrolyzer. The fuel phase separator 303separates the bulk of the liquid product from the gaseous fuel stream.The liquid product level in the fuel phase separator 303 is reported tothe control system by level gauge (LG61). The fuel phase separator isalso equipped with a pressure transducer (PT65).

The liquid product is preferably transferred periodically to theoxidizer side of the electrolyzer stack 115. This liquid producttransfer is performed by opening the product-reduction line valve 355and closing the product-output line valve 353, thus allowing the productoutput line pump 401 to pump the liquid product from the fuel phaseseparator 303 regardless of the pressure differential between theoxidizer and fuel sides of the electrolyzer stack. A product-reductionline check valve 477 prevents accidental back flow in theproduct-reduction line. When the liquid product level in the fuel phaseseparator 303 reaches a preset low limit, the product-output line valve353 is opened and the product-reduction line valve 355 is closed, andthe system resumes normal operation.

An ion-exchange column 481, situated on the side of the oxidizer phaseseparator 301, continuously removes metal ions from the liquid productreceived and stored in the oxidizer phase separator, which helps prolongthe life of the electrolyzer stack 115. An ion-exchange-column pump 483is used to circulate the liquid product from the oxidizer phaseseparator through the ion exchange column. A level sensor (LG41)monitors the liquid product level in the oxidizer phase separator tankfor the control system.

The electrolyzer effectively acts as a compressor and “pumps” thegaseous oxidizer and fuel up to a high pressure (e.g., 400 psig) duringthe electrolyzer charging cycle. Under the pumping action of theelectrolyzer, the oxidizer phase separator passes the received humid,gaseous oxidizer back through the bidirectional oxidizer line 325.Likewise, the fuel phase separator passes the received, humid, gaseousfuel back through the bidirectional fuel line 321.

Before entering the gaseous fuel and oxidizer storage tanks, the humidfuel and oxidizer streams are dried to a dew point below −20° C. This isaccomplished by cooling the gases using a freeze-dry methodology. Inparticular, the bidirectional drier 311, which is a heat exchanger, usescold liquid flow from the ram-air heat exchanger 309. Optionally, thelower part of the drier cools the gases to temperatures above freezing,allowing the majority of the water vapor to condense out on the drierwalls and flow back into the phase separator. The upper part of thedrier is operated at temperatures well below freezing, such that most ofthe remaining water vapor is frozen out of the drier walls. Sufficientvolume is provided in the drier to accommodate one charge cycle worth ofice buildup without risking clogging of the gas flow path. The drier canbe unitary, or it can be two units−one on each bidirectional line. Thislightweight, high efficiency drier is preferably configured to preventice from clogging the flow path and plugging the drier.

The control system controls the temperature of the inlet stream usingtwo proportional bidirectional-drier loop valves 407 and 409 and asignal from a temperature sensor (T6), which is on the inlet of the sideof the bidirectional drier 311. A flow gauge (FG3) and a temperaturesensor (T5) are used to calculate the efficiency of the coolingoperation. Temperature sensors (T44) and (T64) measure the temperaturesat the exit of bidirectional drier 311 for the dried gaseous oxidizerand the dried gaseous fuel, respectively, and are used to verify theproper operation of the bidirectional drier.

The dried gaseous oxidizer and dried gaseous fuel are preferably storedat pressures up to 400 psig in the storage tanks. The oxidizer tank 217is equipped with a pressure relief valve 411. This valve is set at themaximum acceptable tank operating pressure and will open in an event ofa malfunction of the pressure control system to prevent tank rupture ifthe pressure rises above the preset limit. An oxidizer-vent solenoidvalve 412 can be used for venting of the reactant. An bidirectionaloxidizer line solenoid valve 413 is used by the control system toisolate the oxidizer tank 217 in an event of a malfunction in otherparts of the system. A gas sensor (GS41) monitors the concentration offuel in the oxidizer tank for the control system.

Likewise, the fuel tank 215 is equipped with a pressure relief valve415. This valve is set at the maximum acceptable tank operating pressureand will open in an event of a malfunction of the pressure controlsystem to prevent tank rupture or explosion. A fuel-vent solenoid valve417 can be used for venting of the reactant. A bidirectional fuel linesolenoid valve 419 is used by the control system to isolate the fueltank in an event of a malfunction in other parts of the system. A gassensor (GS61) monitors the concentration of the oxidizer in the fueltank.

Detailed Operation Of The Fuel Cell (Discharging)

During fuel cell operation, the gaseous oxidizer flows from the oxidizertank 217, through the oxidizer solenoid valve 413, the bidirectionaldrier 311, the oxidizer phase separator 301, an oxidizer-input-linesolenoid valve 431, a flow gauge (FG41), the oxidizer side of the fuelcell stack 117, a product-output-line pump 433, a product-output-linecheck valve 435, and back to the oxidizer phase separator 301. The fuelcell can operate at tank pressures up to 400 psi and as such does notrequire complex and heavy gas pressure regulation at the fuel cellinlets. A product-output-line solenoid valve 437 is used to vent/purgethe oxidizer side of the stack when needed.

Liquid product from the oxidizer phase separator 301 is used as acooling fluid for the fuel cell. In this case, liquid product from theoxidizer phase separator flows through the main heat exchanger 307, theproduct-output line valve 353, the product-output line pump 401, afuel-cell-cooling-line proportional valve 439, a fuel-cell cooling linefilter 441, a flow gauge (FG21), the cooling plates of the fuel cellstack 117, and back to the oxidizer phase separator tank. Afuel-cell-cooling-line check valve 443 prevents back flow in an event ofa malfunction. Pressure sensors (PT21) and (PT23), and temperaturesensors (T22) and (T23) are used to control the flow through the coolingloop of the fuel cell stack 117.

Fuel from the fuel tank 215 flows through the fuel cell stack 117 viathe solenoid valve (SV63), the bidirectional drier 311, the fuel phaseseparator 303, a fuel-input-line solenoid valve 451, a flow gauge(FG61), the fuel side of the fuel cell stack, a fuel-input-line pump453, a fuel-input-line check valve 455, and then returns to the fuelphase separator. A solenoid valve 457 is used to controllably purge thefuel side of the fuel cell stack.

The gaseous fuel and gaseous oxidizer, which are stored at very lowtemperature (e.g., −40 degrees C. or less) in the storage tanks, arefirst heated up to temperatures above 0 degrees C. in the device thatwas used for drying them during the day, and then they are humidified inthe fuel and oxidizer phase separators.

During fuel cell operation a small amount of liquid product may make itsway to the fuel phase separator 303 as a vapor or small droplets carriedwith the fuel stream. This liquid product condenses and is collected atthe bottom of the fuel phase separator and sent back to the oxidizerphase separator 301 periodically, by opening the product-reduction linevalve 355 and closing the product-output line valve 353, regardless ofthe pressure differential between the fuel and oxidizer side of thesystem.

The liquid product from the fuel cell stack 117 is stored in theoxidizer phase separator 301. The liquid product exits the fuel cellstack with the excess oxidizer stream in vapor and liquid form. Theoxidizer phase separator 301 serves as a phase separator and preventsliquid product from reaching the fuel cell stack inlet. The ion-exchangecolumn 481 is used as needed and the ion-exchange-column pump 483 iscontrolled based upon a signal from a liquid product conductivity probe(C21). A liquid product conductivity probe (C22) is used to assess theefficiency of the ion-exchange bed. No change in the conductivity of theliquid product at nominal flow condition would indicate that it is timeto replace the ion-exchange resin in the column.

During fuel cell operation, the bidirectional drier is used to preheatthe cold gases coming from the gaseous fuel and oxidizer tanks. In thiscase—warm coolant is sent to the drier, bypassing the ram-air heatexchanger 309. During fuel cell operation the bidirectional drier iswarmed above freezing, which melts the frozen product from prior dryingoperations, and leading to water flowing back into the phase separator.The bidirectional-drier loop valves 407 and 409 may be fully closed andfully open to entirely bypass the ram-air heat exchanger 309. Themain-loop proportional valves 403 and 405 are used to control thetemperature of the coolant on the shell side of the main heat exchanger307.

While a particular form of the invention is illustrated and described,it will be apparent that various modifications can be made withoutdeparting from the spirit and scope of the invention. Thus, although theinvention is described in detail with reference only to the preferredembodiment, those having ordinary skill in the art will appreciate thatvarious modifications can be made without departing from the invention.

Structure

The energy storage system design utilizes lightweight compositestructures for the primary pod structure, tanks, and fuel cell andelectrolyzer stack end plates. Lightweight polymer materials are used inthe fuel cell, electrolyzer, and tank design. Using the drier to dry thehydrogen and oxygen gases before they are stored in their respectivetanks preferably eliminates the need for a heavy thermal control systemon the tanks. The parts of the system that contain water are housed in alightweight, thermal enclosure that keeps the water from freezingthroughout the operational cycle.

Lightweight composite tanks with low-temperature polymer liners are usedto store the dry oxygen and hydrogen gases. Polymer liners integral withthe composite shell are used to prevent leaks of hydrogen and oxygenfrom the system. These are preferably, as composite tanks with metalliners are likely to have a significant weight cost for the preferredaircraft applications.

From the above description, it should be clear that a single liquidproduct system provides a variety of functions. Stack product managementis achieved by using phase separation tanks to collect water from thefuel cell stack exhaust streams. Product can be transferred from thefuel side to the oxidizer side of the system. The product is also usedto cool the fuel cell during operation. During electrolyzer operation,the product is fed into the electrolyzer for dissociation into oxidizerand fuel. The aircraft can be fueled by simply adding product (e.g.,water) to the oxidizer phase separator at the start of the flight.

1. A closed loop energy storage system comprising: a fuel cell systemconfigured to react a gaseous fuel with a gaseous oxidizer to produce aliquid product and electrical power, the fuel cell system being furtherconfigured to separate the liquid product into a humid fuel includingthe gaseous fuel and product humidity, and a humid oxidizer includingthe gaseous oxidizer and product humidity, wherein the humid fuelproduced by the fuel cell system is intermixed with liquid product, andwherein the humid oxidizer produced by the fuel cell system isintermixed with liquid product; a first phase separator configured toseparate the liquid product from the humid fuel; a second phaseseparator configured to separate the liquid product from the humidoxidizer; a fuel tank configured to store the gaseous fuel forsubsequent reaction in the fuel cell system; an oxidizer tank configuredto store the gaseous oxidizer for subsequent reaction in the fuel cellsystem; and a heat exchanger configured to preheat the gaseous fuel fromthe fuel tank and the gaseous oxidizer from the oxidizer tank prior tothe gaseous fuel and the gaseous oxidizer being reacted in the fuel cellsystem; wherein the fuel cell system is configured to receive thegaseous fuel from the fuel tank and the gaseous oxidizer from theoxidizer tank; and wherein the heat exchanger is configured to receiveheat from the fuel cell system, and is further configured to use thatheat to concurrently preheat the gaseous fuel and the gaseous oxidizer.2. The energy storage system of claim 1, and further comprising acontroller programmed to control the operation of the fuel cell system,the heat exchanger, the first and second phase separators, the fuel tankand the oxidizer tank such that during operation of the fuel cellsystem, the heat exchanger receives heat from the fuel cell system andpreheats both the gaseous fuel and the gaseous oxidizer.
 3. The energystorage system of claim 1, wherein the fuel cell system includes a fuelcell stack and a separate electrolyzer stack.